1. Field of the Invention
The present invention relates to a method of making an SiC single crystal and apparatus for making an SiC single crystal in which high-quality SiC suitable for semiconductor electronic components is grown.
2. Related Background Art
Being a material excellent in resistance to chemicals such as acids and alkalis, less likely to be damaged by high energy radiation, and yielding a high durability, SiC has been used as a semiconductor material.
In order for SiC to be used as a semiconductor material, it is necessary to obtain a high-quality single crystal thereof having a certain order of dimensions. Conventionally utilized as a method of growing an SiC single crystal of the aimed scale is Acheson method employing a chemical reaction or Lely method employing sublimation/recrystallization technique.
In particular, as a method of growing a bulk of SiC single crystal, Japanese Patent Publication No. 59-48792, for example, discloses so-called modified Lely method in which, in a crucible made of graphite, an SiC single crystal of appropriate dimensions is used as a seed crystal, and material SiC powder is sublimed in an atmosphere under a reduced pressure, so as to be recrystallized on the seed crystal, whereby an SiC single crystal of the aimed scale is grown.
Of the above-mentioned conventional methods, the Acheson method heats a mixture of silica and coke in an electric furnace and deposits the crystal due to naturally occurring nucleation, thus yielding a large amount of impurities and making it difficult to control the form of resulting crystal and crystal faces, whereby it is hard to produce high-quality SiC single crystals.
Also, in the case where an SiC single crystal is made by the Lely method, since the crystal is grown due to naturally occurring nucleation, it is difficult to control the form of crystal and crystal faces.
On the other hand, an SiC single crystal having a considerably good quality can be obtained in accordance with the invention disclosed in the above-mentioned Japanese Patent Publication No. 59-48792, which belongs to the modified Lely method. When the SiC single crystal is obtained by this method, however, SiC crystals naturally occur from the graphite crucible during the crystal growth period. Using these SiC crystals as nuclei, crystals rapidly grow and inhibit the crystal growth from the seed crystal, thus making it difficult to yield a crystal with a high homogeneity.
Further, there is a problem that, under the influence of heat radiation, the temperature of the upper face of the material becomes higher than that within the material, whereby the amount of sublimation is large at the early stage of growth and gradually decreases as the surface is graphitized. In order to overcome this problem, Japanese Patent Application Laid-Open No. 5-105596 proposes to make a material contain carbon and further form a surface portion of the material with a layer containing carbon, thereby preventing heat radiation from occurring from the upper part of the crucible. Even in this method, however, it is difficult to effect such control that the material consistently reaches the seed crystal under the same vapor pressure during the synthesis, whereby the production of a high-quality SiC single crystal cannot be expected.
Also, the area where the material is sublimed upon heating by the heat conduction or heat radiation from the crucible gradually expands from the material in the vicinity of the part in contact with the side face or bottom face of the crucible to the material located at the center part. Since the part of material in the vicinity of the side face or bottom face of the crucible sublimed in the early stage changes into highly heat-insulating soot-like powder as the sublimation area expands, however, the heat conduction and heat radiation to the material at the center part would decrease drastically, whereby the sublimation of the material at the center part may diminish suddenly or fail to occur. In particular, for synthesizing a single crystal having a large area, the crucible for charging the material is required to have a large diameter as well, whereby the radial alteration of material would be a severe problem. Though the growth apparatus disclosed in Japanese Patent Application Laid-Open No. 5-58774 aims at uniformly heating the material by installing a heat conductor within the crucible, it cannot restrain the SiC material from subliming so as to change into soot-like powder, thus failing to keep crystallizing speed from changing over time in principle, whereby the manufacture of high-quality SiC single crystal cannot be expected in this apparatus, either.
FIG. 5 is a graph showing vapor pressure curves of carbon (C) and SiC, in which the ordinate (on a logarithmic scale) and the abscissa indicate pressure (Pa) and temperature (xc2x0C.), respectively. As shown in FIG. 5, the vapor pressure of Si is higher than that of SiC2 or Si2C occurring during the generation of SiC by one digit. For enhancing the SiC-forming speed, it is necessary to supply a sufficient amount of Si and C to the seed crystal. In this case, however, there is a problem that, if the material temperature is raised so as to increase the partial pressures of SiC2 and Si2C, which have low vapor pressures, in order to sufficiently supply C, the partial pressure of the Si system will be so high that the stoichiometry (stoichiometric composition) of the material and synthesized crystal may shift.
WO9713013A discloses an epitaxial growth method in which a high-speed jet of silane gas is sprayed onto an SiC substrate within high-temperature hot walls. The SiC single crystal can be grown at a high speed in this technique. Since Si is supplied by a gas, however, there occurs a problem that hydrogen etches SiC within the high-temperature hot walls. Also, the silane gas may form particles in the vapor phase, thus contaminating the inside of the apparatus and degrading the SiC single crystal.
FIG. 6 shows the temperature dependence of Si partial pressure in a major reaction in which SiC grows in thermal CVD of SiC. From this graph, it can be seen that, as the hydrogen partial pressure rises, the reverse reaction for SiC growth proceeds, whereby SiC is etched. Namely, when the hydrogen partial pressure is high, it becomes difficult to form a high-quality SiC single crystal.
In view of such conventional problems, it is an object of the present invention to provide a method of making an SiC single crystal and an apparatus for making an SiC single crystal in which a high-quality SiC single crystal can be obtained.
In order to overcome the above-mentioned problems, the present invention provides a method of making an SiC single crystal, the method comprising a disposing step of disposing solid Si with in a first temperature area T1, and disposing a seed crystal of SiC within a second temperature area T2 that is higher than the first temperature area T1; an Si-evaporating step of evaporating Si from the first temperature area T1; an SiC-forming-gas-generating step of generating an SiC-forming gas by reacting thus evaporated Si and carbon; and a single-crystal-forming step of causing the SiC-forming gas to reach the seed crystal so as to form the SiC single crystal.
First, in the method of making an SiC single crystal in accordance with the present invention, solid Si is evaporated as being heated by the first temperature area T1. Here, as the temperature of the first temperature area T1, is regulated, the partial pressure of Si can be adjusted. Subsequently, thus evaporated Si is reacted with carbon, where by an SiC-forming gas is generated. As the SiC-forming gas reaches the seed crystal of SiC, the SiC single crystal is formed. Here, if the partial pressure of carbon to combine with the evaporated Si is made substantially the same as the partial pressure of Si determined by the temperature of the first temperature area T1, a high-quality SiC single crystal can be obtained.
Also, since a solid source of Si is used, the partial pressure of hydrogen in the atmosphere decreases, thereby eliminating the problem that the SiC single crystal is etched. Further, since unstable gases such as silane are not used as the SiC source, there would be no problem of particles being formed upon decomposition of the gases in the vapor phase. As a consequence, Si can sufficiently be supplied, so as to enable high-speed growth and make it possible to prevent the SiC single crystal from degrading due to the particles.
Here, it can be seen from FIG. 6 that, as the hydrogen partial pressure decreases, the reaction proceeds in the direction causing SiC to grow. Since the flux amount (cmxe2x88x921sxe2x88x921) of Sic can be calculated from its partial pressure, assuming that all of the flux contributes to growth, it can be seen that a growth rate as high as several hundred xcexcm/h is expectable.
Preferably, in the method of making an Sic single crystal in accordance with the present invention, solid carbon is disposed in a third temperature area T3 at a temperature higher than that in the second temperature area T2 in the disposing step; the Si-forming gas is formed by causing the Si evaporated in the Si-evaporating step to pass through the third temperature area T3 and react with carbon in the SiC-forming-gas-generating step; and the SiC-forming gas is caused to reach the seed crystal in the single-crystal-forming step to form the SiC single crystal.
Namely, in this case, the partial pressure of Si can be adjusted by regulating the temperature of the first temperature area T1, and the partial pressure of carbon can be made substantially the same as that of Si by regulating the temperature of the third temperature area T3. In general, in order for the partial pressure of carbon and the partial pressure of Si to become identical to each other, it is necessary for carbon to have a temperature higher than that of Si. Here, since the temperatures of Si and carbon are raised independently of each other, a sufficient amount of carbon can be supplied to the seed crystal while suppressing the amount of evaporation of Si. As the seed crystal, a single crystal substrate of SiC may also be used.
FIG. 7 is a graph showing the respective vapor pressure curves of Si and C, in which the ordinate (on a logarithmic scale) and the abscissa indicate pressure (Pa) and temperature (xc2x0C.), respectively. As can be seen from FIG. 7, as Si which can yield a sufficient vapor pressure at 1400xc2x0 C. and over is caused to pass through the area of C (graphite) heated to 2000xc2x0 C. or higher, the SiC single crystal can be synthesized with a favorable controllability.
Also, highly pure materials are inexpensively available for solid Si and C (graphite), which are raw materials, respectively. Therefore, the concentration of impurities in the SiC single crystal being synthesized can greatly be lowered. Further, since each of material Si and C is a single element, unlike the case using SiC powder, no composition changes would occur during synthesis, whereby the synthesizing condition becomes stable, thus allowing a high-quality SiC single crystal to be obtained. Also, since the materials at the time of filling are not in the form of powder but solid (bulk) of Si and graphite, the filling ratio is so high that a large, elongated SiC single crystal can be synthesized.
Silicon Carbide-1973, p. 135 (Proceedings of the Third International Conference on Silicon Carbide held at Miami Beach, Fla. on Sep. 17-20, 1973) discloses that an SiC single crystal with a good quality was obtained when the temperature of molten Si was 2200xc2x0 C. in the state where the growth chamber in which SiC was formed had a temperature of 2500xc2x0 C. The heater used in the growth apparatus disclosed in this literature, however, had only one zone, and the temperature of molten Si was not forcibly adjusted to but only turned out to be 2200xc2x0 C., thus being greatly different from the present invention in this regard.
Preferably, in the present invention, a shield made of carbon, quartz, or SiC is disposed at a boundary between Si in a liquid phase and Si in a vapor phase in the first temperature area T1, so as to control the vapor pressure of Si. Here, as carbon, glass-like carbon (glassy carbon) is suitable in particular, which forms an excellent shield without reacting with Si even at a high temperature.
Preferably, the above-mentioned shield and solid carbon in the third temperature area T3 are mechanically connected to each other, such that, as the shield changes its position along with a decrease in the amount of Si caused by evaporation of Si in the first temperature area T1, carbon migrates so that the distance between the formed SiC single crystal and carbon is kept substantially constant. Employing such a configuration enables an SiC single crystal to be formed stably for a long period of time.
Preferably, solid carbon in the third temperature area T3 is formed with a through hole through which evaporated Si can pass. Using carbon formed with a through hole such as capillary or slit would increase the area at which evaporated Si comes into contact with carbon, thus allowing Si and carbon to react with each other efficiently.
Preferably, in the method of making an SiC single crystal in accordance with the present invention, Si and the seed crystal of SiC are disposed in a gas containing carbon in the disposing step, the evaporated Si is reacted with a carbon component in the gas to form the Sic-forming gas in the SiC-forming-gas-generating step; and the SiC-forming gas is caused to reach the seed crystal in the single-crystal-forming step, so as to form the SiC single crystal.
Namely, in this case, the partial pressure of Si can be adjusted by regulating the temperature of the first temperature area T1, and the partial pressure of carbon can be made substantially the same as that of Si by regulating the amount of supply of the gas containing carbon, whereby a high-quality SiC single crystal substrate can be obtained. Here, as the seed crystal, a substrate of single crystal SiC may be used.
Preferably, a shield made of carbon, quartz, or SiC is disposed at the boundary between Si in the liquid phase and Si in the vapor phase in the first temperature area T1, so as to control the vapor pressure of Si. Here, as carbon, glass-like carbon (glassy carbon) is suitable in particular, which forms an excellent shield without reacting with Si even at a high temperature.
Preferably, an argon gas is used as a carrier gas of the Si evaporated from the first temperature area T1. Using the argon gas as the carrier gas can prevent by-products from being generated.
Preferably, in the single-crystal-forming step, the seed crystal is rotated at 100 rpm or over. Thus rotating the seed crystal at a high speed can minimize the film thickness distribution and further enables high-speed growth. It is due to the fact that the rotation would thin the diffusion layer of the substrate surface, thereby increasing the driving force for diffusion. As a consequence, the growth rate can be increased without using a proximity method such as sandwich technique.
The apparatus for making an SiC single crystal in accordance with the present invention comprises an Si-disposing section in which solid Si is disposed; a seed-crystal-disposing section in which a seed crystal of SiC is disposed; a synthesis vessel adapted to accommodate the Si-disposing section, the seed-crystal-disposing section, and carbon; heating means adapted to heat the Si-disposing section and the seed-crystal-disposing section; and a control section for transmitting to the heating means a command for heating the Si to an evaporation temperature of Si or higher and heating the seed crystal to a temperature higher than that of the Si, wherein the Si evaporated by the heating means is adapted to reach the seed-crystal-disposing section.
In the apparatus for making an SiC single crystal in accordance with the present invention, Si is evaporated by the heating means receiving the command from the control section. Here, as the heating temperature of Si is regulated, the partial pressure of Si can be adjusted. Subsequently, thus evaporated Si is reacted with carbon, whereby an SiC-forming gas is generated. Then, as the SiC-forming gas reaches the seed crystal disposed in the seed-crystal-disposing section, the SiC single crystal is formed. Here, if the partial pressure of carbon is made substantially the same as the partial pressure of Si, then a high-quality SiC single crystal can be obtained.
Preferably, in the apparatus for making an SiC single crystal in accordance with the present invention, solid carbon is disposed between the Si-disposing section and seed-crystal-disposing section in the synthesis vessel, the control section controls the heating means such that the temperature of solid carbon becomes higher than that of the seed crystal, and the Si evaporated by the heating means is adapted to reach the seed crystal by way of the solid carbon.
When such a configuration is employed, the partial pressure of Si can be adjusted by regulating the heating temperature of Si, and the partial pressure of carbon can be made substantially the same as that of Si by regulating the heating temperature of carbon, whereby a high-quality SiC single crystal substrate can be obtained.
Preferably, the apparatus for making an SiC single crystal in accordance with the present invention is configured such that the synthesis vessel is adapted to accommodate a gas containing carbon, the Si evaporated by the heating means reacts with carbon in a carbon component in the gas so as to generate the SiC-forming gas, and the SiC-forming gas is adapted to reach the seed crystal.
When such a configuration is employed, the partial pressure of Si can be adjusted by regulating the heating temperature of Si, and the partial pressure of carbon can be made substantially the same as that of Si by regulating the amount of supply of the gas containing carbon to the synthesis vessel, whereby a high-quality SiC single crystal substrate can be obtained.
Preferably, in the apparatus for making an SiC single crystal in accordance with the present invention, an inner face of the synthesis vessel is formed from diamond-like carbon or glass-like carbon. This can suppress natural nucleation in the inner face of the synthesis vessel, thus allowing a high-quality SiC single crystal to be synthesized.
Preferably, a heat shield made of graphite is disposed outside the synthesis vessel. This can suppress the heat dissipation caused by heat radiation.
Preferably, the heat shield is made of a plurality of rectangular graphite sheets disposed close to each other with a gap therebetween, such as to yield substantially a cylindrical form as a whole. This can suppress the induced current caused by high-frequency heating. Further, if a plurality of such heat shields are disposed radially of the synthesis vessel, then the heat dissipation and induced current can further be suppressed.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.